In recent years, continuously variable transmissions (hereinafter occasionally referred to merely as “CVT”) have been extensively used as devices for power transmissions. The CVT is a transmission capable of continuously changing an output rotation speed relative to a constant input rotation speed, and various types of CVT are known in the art. The CVT is typically classified into a friction drive type in which a power is transmitted by means of a metallic belt or chain, and a traction drive type in which a power is transmitted without using such an element.
Comparing the friction drive type CVT with the traction drive type CVT, the traction drive type CVT is generally capable of transmitting a large capacity of power nevertheless its small scale. For this reason, the application fields of the traction drive type CVT are continuously expanded. For example, power transmissions having a so-called traction drive type transmission mechanism which include not only continuously variable speed-reducing or speed-increasing devices for industrial equipments and continuously variable transmissions for automobiles but also transmissions into which a principle of the traction drive or a part thereof is incorporated, have been increasingly put into practice.
Further, the power transmissions having a traction drive type transmission mechanism have been recently utilized in more extensive application fields such as airplanes and helicopters, and practically used, for example, for controlling a rotation speed of generators of airplanes or rotors of helicopters.
Lubricants for power transmission, in particular, those for power transmission having a traction drive type transmission mechanism, are required to have a high coefficient of traction for enhancing a transmission performance thereof. As the lubricants satisfying such a requirement, there have been proposed alicyclic hydrocarbon compounds or naphthene ring-containing esters. Examples of the alicyclic hydrocarbon compounds include dicyclohexyl compounds such as typically 2-methyl-2,4-dicyclohexyl pentane, and dimerized norbornanes (refer to Japanese Patent Application Laid-open Nos. 7664/1972 and 95295/1991), whereas examples of the naphthene ring-containing esters include those compounds such as typically an ester of cyclohexanol and cyclohexanecarboxylic acid, a diester of cyclohexanecarboxylic acid and neopentyl glycol and an ester of succinic acid and cyclohexanol (refer to Japanese Patent Publication Nos. 31373/1994, 31365/1994, 31366/1994, 74350/1995, 74351/1995 and 293265/1999).
The alicyclic hydrocarbon compounds have advantages such as a high power transmission performance because of a high coefficient of traction thereof, but tend to exhibit a slightly low flash point. On the other hand, the naphthene ring-containing esters have a relatively high flash point, but tend to be insufficient in coefficient of traction. Therefore, in the application fields requiring especially a high safety against firing, for example, when applied to power transmissions having a traction derive type transmission mechanism including a traction drive type CVT mechanism for controlling a rotation speed of generators of airplanes and rotors of helicopters, any of these compounds tends to be unsatisfactory as a lubricant therefor.
For these reasons, it has been required to provide lubricants for power transmission which exhibit not only a high coefficient of traction but also a high flash point.